In a computer mother board, a RTC(Real-Time Clock)is used for maintaining the data stored in a CMOS. Generally, electrical power is provided to the RTC by a battery arranged on the mother board. If the battery is used up, the data in the RTC CMOS RAM are lost, and consequently the computer cannot boot through the hard disk. If this happens and a new battery is mounted, the data in the RTC CMOS RAM should be set again. It is troublesome to re-set the data. In general the user does not know the function of the battery. In other words, when the computer cannot be booted and a computer technician is called, the technician has no way to evaluate the state of the computer. The technician can only find the causes of failing to boot by using a number of tools to test the functions of the computer one by one, which bothers the customer. When the power of the battery is used up and the computer cannot boot, if the user cannot repair it by himself, then the user can at least tell the warning message to the technician. The technician may then repair the computer by directly replacing a new battery. Thus, the problem may be solved, the maintaining time is shortened and the maintaining cost decreased.